1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly relates to a semiconductor device capable of reducing an off-leakage current of a transistor.
2. Description of Related Art
Demands for power consumption reduction of semiconductor devices have been increased in recent years. One of techniques for reducing power consumption of semiconductor devices is a power supply hierarchization technique. The power supply hierarchization technique is a technique in which a power supply wiring for supplying power to a circuit block is hierarchized into a main power supply wiring and a sub power supply wiring, thereby reducing an off-leakage current (a sub-threshold current) during a standby state (see Japanese Patent Application Laid-open No. 2000-195254).
Specifically, the main power supply wiring and the sub power supply wiring are short-circuited in an active state, thereby supplying power to these power supply wirings. An operating voltage is thus supplied correctly to corresponding circuit blocks. Meanwhile, during a standby state, the main power supply wiring and the sub power supply wiring are disconnected from each other, thereby blocking the power supply to the sub power supply wiring. With this arrangement, power supply to transistors that are irrelevant to maintaining a predetermined fixed logic is stopped. In this way, an off-leakage current of a circuit block whose logic is fixed during a standby state, such as a main word driver of a DRAM (Dynamic Random Access Memory), can be reduced.
Japanese Patent Application Laid-open No. 2000-195254 only discloses a technique of determining whether a power supply of a main power supply wiring is supplied to a sub power supply wiring in a standby state and an active state of a semiconductor device. However, in view of the entire power consumption of a semiconductor device during a standby state, an off-leakage current of a transistor that constitutes a circuit block is a static current and its power consumption varies depending on temperature. Meanwhile, a switching current of a power-supply control transistor that connects a main power supply wiring to a sub power supply wiring is a dynamic current and its power consumption hardly varies depending on temperature. For example, according to an MOS transistor, which is a representative transistor, its off-leakage current is reduced as the temperature decreases. Besides, the switching current of a power-supply control transistor has a low temperature dependency because a current generated by charging/discharging a gate capacity of the power-supply control transistor is dominant. When a chip temperature is lower than a predetermined temperature, power consumption by a charge and discharge current in a signal line that drives the power-supply control transistor becomes larger than that by an off-leakage current. In such a temperature region, the entire power consumption of the semiconductor device during the standby state is reduced if the power-supply control transistor is always turned on (conductive) to activate a circuit constituted by a transistor.
The temperature of a semiconductor device changes rapidly depending on an operating state of the semiconductor device itself or an adjacent semiconductor device. When the semiconductor device is a memory, in a case of refreshing memory cells at a predetermined period for maintaining information about the memory cells, successive accesses are made in a state before the refresh operation and thus the temperature (silicon substrate's temperature) can be increased, or the state before the refresh operation is maintained in a standby state and thus the temperature can be maintained low. Heat from other silicon chips can be shared by a MCP or the like. That it, the value of an off-leakage current generated in a circuit block always changes depending on continuously varying temperature.
In recent years, the operating voltage of a semiconductor device has been gradually reduced for power consumption reduction. A significantly low voltage, such as around 1.0 V of an external supply voltage, has been used recently and an internal supply voltage to be supplied to a circuit block has been also further reduced correspondingly. When the operating voltage of the circuit block is reduced, the threshold voltage of a transistor needs to be further reduced to maintain the operating voltage of the circuit block. Therefore, there is a problem that the off-leakage current of a transistor in a non-conductive state is further increased.
Besides, semiconductor devices usually include an internal supply voltage (positive or negative boost internal voltage) generated within the semiconductor devices regardless of a decrease in an external supply voltage. A circuit block to which the internal supply voltage is supplied also has the above described problems about the off-leakage current and the power-supply control transistor.
In view of such circumstances, even various circuit blocks, with which an off-leakage current during a standby state had not been problematic before, need to apply the power supply hierarchization technique.
However, the temperature characteristic of an off-leakage current of a transistor can be different for each circuit block. In this case, a threshold temperature at which the charge and discharge current of a power-supply control transistor is larger than the off-leakage current thereof is also different for each circuit block. Therefore, when a threshold temperature for stopping power supply control is set for a plurality of circuit blocks (set uniformly), the power consumption of the entire semiconductor device cannot be reduced sufficiently.